Holograms embossed into metal surfaces

ABSTRACT

A product whereby diffraction patterns and holograms directly onto surfaces of metal material, such as aluminum. Products include household aluminum foil with embossed holograms, and aluminum beverage cans with an embossed hologram on its outside.

This is a division of application Ser. No. 891,544, filed July 29, 1986,now U.S. Pat. No. 4,725,111.

BACKGROUND OF THE INVENTION

This invention relates generally to implementation of holography, andmore specifically to the replication of holograms by embossing.

Mass replicated holograms are commonplace, appearing on credit cards,consumer packaging, and the like. The holograms allow viewing of imagesformed in reflective light at locations other than the hologram surface.This can be in the form of a three-dimensional image of an object or ofa complex image having different planes.

Such holograms are made by interfering two beams of coherent light at afinite angle with each other on a photosensitive medium. One of thebeams interacts with an object whose image is to be recorded, and theother is a reference beam. An image of the object is focused into ornear the surface of the resulting hologram by appropriate opticalelements, which may include use of another, intermediate hologram. Theresulting master hologram is made to be of surface relief type; that is,the image information is stored in surface variations.

The fragile photographic hologram master is then used to make a rigidembossing master plate in order to emboss replicas. Such a rigid masteris formed in an electrolytic bath on the surface of the photographicmaster. Multiple masters are then made from this first master, for usein embossing holograms on substrate material. The metal masters aregenerally made of nickel.

The most common substrate materials into which holograms are embossed bysuch masters include Mylar, a polyester, or similar sheet plasticmaterial. Usually, such material has been coated with a thin layer ofreflective material, such as aluminum, in order to result in areflective hologram. The aluminum is usually vapor deposited onto thesubstrate in a thickness merely sufficient to provide reflectiveproperties to the substrate.

The embossing process involves heating the aluminized substrate to atemperature at which it becomes plastic, and then the nickel masterplate is forced against the substrate to imprint the surface reliefpattern in it. The embossing is usually done through the aluminumcoating, but it is the substrate itself that, upon cooling, holds thesurface relief pattern. The aluminum layer follows the surface reliefpattern and thus is capable of reconstructing the originalholographically recorded image in reflected light.

In actual use, these replicated holograms are usually subsequentlyattached to another surface, such as that of a credit card blank. Suchattachment is made by use of an ordinary adhesive or by use of hotstamping techniques.

It is a primary object of the present invention to develop techniquesfor embossing holograms onto a broader range of substrates.

SUMMARY OF THE INVENTION

This and additional objects are accomplished by the various aspect ofthe present invention, wherein, briefly, a metal master hologram is usedto emboss a surface relief pattern directly onto a metal surface,thereby producing a reflection hologram in a single step. An example isthe embossing of holograms on aluminum cans. Another example is theembossing of aluminum wrapping foil that is sold in grocery stores foruse in food storage. Both of these are examples of currently usefularticles having holograms embossed directly on their surfaces in orderto provide decoration or display information, all without having tofirst replicate the hologram in sheet plastic material that must besubsequently attached to such articles in a separate step.

A hologram is treated herein as a particular type of diffractiongrating. The same techniques as summarized above also apply to theembossing of a diffraction pattern that does not carry a holographicallyrecorded image. Such a diffraction pattern may be used in certainapplications where it is desired to develop a decorative color spread ofreflected light without reconstructing an image. In either case, thesurface relief pattern that is embossed onto the metal surface is veryfine since it was formed by the interference of two beams of light. Itis the fineness of the surface relief pattern that makes the embossingof it have some difficulties that are not experienced when embossingaluminum and other metals with much coarser patterns.

Additional advantages and features of the various aspects of the presentinvention will become apparent from the following description of itspreferred embodiments, which description should be taken in conjunctionwith the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the construction of a hologram or diffraction elementby interfering light beams;

FIG. 2 shows a portion of a surface relief metal embossing master;

FIG. 3 illustrates generally the embossing of aluminum or other metalsaccording to the present invention;

FIG. 4 illustrates generally the embossing of aluminum or other metalcans according to the present invention;

FIG. 5 shows exemplary yield strength characteristics as a function oftemperature for a master embossing plate and material being embossed;

FIG. 6 illustrates a completed embossed aluminum foil product; and

FIG. 7 illustrates a finished embossed aluminum can product.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring initially to FIG. 1, a standard technique is illustrated formaking a hologram or other form of diffraction pattern. A photosensitivemedium (detector) 11 is simultaneously illuminated with mutuallycoherent beams 13 and 15 that intersect the detector 11 at a finiteangle with each other. The result is an interference pattern that isrecorded by the detector 11. If one of the beams 13 and 15 carriesinformation of an object and the other beam does not, the interferencepattern recorded on the photodetector 11 is a hologram capable ofreconstructing an image of the object.

In standard techniques of mass replicating holograms and diffractionpatterns, a metal master is formed from the recorded photodetector 11.An example section 17 of such a metal embossment plate is illustrated inFIG. 2. A surface relief pattern 19 is formed in at least one surface ofthe metal master 17. This surface relief pattern is a recording of theinterference pattern formed at the photodetector 11 in FIG. 1. Thesurface relief pattern is formed, according to one known technique, byelectrolytic deposition of nickel on a photographic surface reliefmaster, such as that made by the technique of FIG. 1. The surface reliefpattern 19 is capable of diffracting light incident on it to reconstructan image, in the case of a hologram, or to diffract the incident lightinto its various colors, in the case of a diffraction pattern.

Such a standard embossing master plate 17 is used to emboss the patternonto a surface of continuous moving sheet material 21, as shown in FIG.3. The sheet material 21 is a metal, aluminum being most easilyembossed. The sheet material 21 can have a rather substantial thicknessor can be extremely thin in the nature of household aluminum foil. Anembossing master 17' is attached to a first rotating drum 23 so that thesurface relief pattern of it contacts the top surface of the sheetmaterial 21 as the drum 23 rotates. A second drum 25, under the sheetmaterial 21, serves to compress the sheet material 21 between the tworollers in order that the surface relief pattern of the metal master 17'is forced against the sheet material 21 with enough pressure to transferthe surface relief pattern to the sheet material. Temperature of thesheet material 21 is increased by a heater 27, preferably containedwithin the roller 23. The roller 25 is preferably cooled (not shown) inorder to prevent calendering of the material.

FIG. 4 schematically illustrates the technique for embossing aluminumcans after they have been formed. A wheel 27 contains a plurality ofmandrels 29-35 held thereby and extending normally away from one surfaceof the wheel. Each of the mandrels is of a shape to hold an aluminum canon it. This type of structure is presently used for applying paint to aformed can. It may also be used to emboss a hologram or diffractionpattern onto the cans. A roller 37 can be positioned to contact each ofthe cans as the wheel 27 is rotated. A metal embossing plate 17" iscarried by an outside surface of the wheel 37. The can is compressedbetween the mandrel 33 and the embossing plate 17", in the positionshown in FIG. 4, to transfer the surface relief pattern onto an outsidesurface of the can. The embossing plate is heated by a heater 39preferably carried within the wheel 37, in order to facilitate theembossing process. Each of the mandrels 29-35 is preferably cooled.

It has been found that there is an optimum temperature range in whichthe aluminum or other metal material is maintained when being embossed.The temperature is most important when the material being embossed isrigid, such as in the case of aluminum having a thickness of 1/16 inchor greater, in order to be able to emboss the surface relief patternwith faithfulness. For very thin, flexible metal foils, temperature isalso important for optimizing the embossing process and thus thebrightness of a reconstructed image from an embossed hologram. Thetemperature need not be raised to the point where the material becomesplastic, as in current techniques that use Mylar, polyesters and thelike for the embossable material, but is preferably raised to within asmall temperature range significantly less than the temperature at whichthe material becomes plastic.

Referring to FIG. 5, a preferred embossing temperature will be explainedwith respect to certain characteristics of the material. The solid curvegenerally illustrates example yield strength characteristics of analuminum alloy as a function of temperature. The dashed curve shows anexample yield strength characteristic of a nickel alloy. For thisexample, it is assumed that the material having the characteristics ofthe dashed outline is used for the embossing master and that that of thecurve of the solid line is the material into which the surface reliefpattern of the embossing master is being impressed. Yield strength ofthe material is a measure of the pressure required to produce apermanent deformation of a certain amount, usually 0.2% of itsthickness. It is a well known, commonly used characteristic by whichmaterials are specified.

It turns out that for most aluminum alloys, and for those of othermetals as well, the yield strength drops dramatically over a narrowincreasing temperature range, while remaining only gradually decreasingwith increasing temperature over adjacent ranges. An optimumtemperature, therefore, is within a range that is approximatelyillustrated at 43 in FIG. 5. This is at the upper end of such a sharplydeclining curve portion. It is optimum because the cost of furtherincreasing temperature in the process brings little by way of a returnof decreasing yield strength of the material being embossed. Conversely,if the temperature is decreased from that range, the yield strengthincreases dramatically and makes it more difficult to emboss thematerial with a faithful reproduction of the surface relief pattern onthe nickel master. It will be noted from the dashed line of FIG. 5 thatthe yield strength of a nickel master is relatively constant over thetemperature range shown and much higher than that of the aluminummaterial being embossed. Of course, the master plate must be many timesless deformable at the embossing temperature chosen than is the materialbeing embossed. It will also be noted from the curves of FIG. 5 that anoperating temperature within the region marked 43 maintainssubstantially a maximum difference in yield strength of the embossingmaster (Ni) and material being embossed (Al), thereby increasing thelife of the embossing master.

As with temperature, there is an optimum pressure range. That pressureis that which will permanently deform the material on the surface beingembossed, but no more. In terms of standard material characteristics,the desired pressure is of a magnitude that is slightly beyond the yieldpoint of the material. A higher pressure will produce little in the wayof a better embossed product and has a disadvantage of reducing the lifeof the embossing master by increased wear and also may cause undesiredcalendering of the sheet material.

Referring to FIG. 6, a short length 45 of standard aluminum foil isillustrated after having a strip of multiple holograms 47 embossed inits surface. The embossed holograms add a decorative feature to anotherwise plain, utilitarian household object. Similarly, a beverage can49 is illustrated in FIG. 7 to have a hologram 51 embossed on itsoutside surface. The hologram 51 can simply contain a decorative image,or, alternatively, can contain a portion of the labeling information.

Although the various aspects of the present invention have beendescribed with respect to its preferred embodiments, it will beunderstood that the invention is entitled to protection within the fullscope of the appended claims.

It is claimed:
 1. A product consisting of a length of thin, flexible aluminum foil and a plurality of holograms embossed directly onto one surface thereof along its length.
 2. A product comprising an aluminum beverage can and a hologram embossed directly onto an outside surface thereof along its length. 